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Strong sewer smell in office building

We have been in our building for 11 years. We have random strong sewer gas smells in the office which seem to happen on a no wind day. The roof has a flat top and our HVAC guys have been up there numerous times trying to figure out what is wrong.
They have raised the vent stacks and installed charcoal filters on them. They have opened up a few walls to modify the vent pipes but the stink continues.
Our building is at the bottom of a valley and weve been told that it looks like the city sewage gas is coming out of our vent stacks creating more than a usual amount of sewage vapor.
No one seems to know what to do.
When I go up there (Im no HVAC professional) I see the following. A flat roof with walls about a foot tall. If the gas is heavy which I believe is true then I see the gas pooling on the roof of the building. We have several Lennox GCS16s with what I believe is a horizontal Economizer Damper on the side. (I just downloaded the manual and that is what I figured the filter on the side was  I may be wrong). If this is an economizer then from what I read it will be sucking in fresh air from the outside or in our case from a pool of stink.
Does my theory make any sense? Your opinions would be most appreciated.

The vents need to be at least 10 ft from the intakes, you can raise them up which looks easiest.
You also need to check any drains to make sure they are not dry.
Typical floor drains, parking lot drains etc.
Use mineral oil in them if they are seldom used.
That will last longer than water and provide a better seal.

Not to put any HVAC people out of work, but a would also get a Master Plumber to 1) look through the original as builts, co-ordinate those prints with any build outs. 2) If there is any location were the sewer smell is dominate check all the drain lines, tie ins above the ceiling grid along with vent piping. 3) Has any of the HVAC contractor's check building static pressure ? If it is positive, does it stay positive ? If it is negative it will suck the fume out of the vents or through dry traps. A very good point about using mineral oil in any drains that are dry and not used. There is a lot more to plumbing then the three basic rules

Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO

First thing I thought of was dry floor drains. I'd like to add don't forget floor drains in the bathrooms. I would have thought they would be separate from the sewer pipes, but we were in one building and traced the smell to that. Ga1279, I'm not a plumber. What are the three basic rules?

I am a Master plumber and a Master HVAC guy and most plumbers I know would not have a clue as to where to start, no offense intended.
I just know I have solved many building issues by looking at all of the issues because I have studied them all.
If he makes it like code requires and goes from their and checks all of his floor drains for being dry he has made a huge step forward!
There is telling what you will find.
I make bets on jobs like these because I always figure them out.
I have not fond one I could not fix!

We have our own fiasco at a national chain restaurant. The are using Trane units with the Economizer intake about a foot or 2 from the roof. The plumbers raised the stacks 6' high, and when you get down near the roof the odor is worse, sewer gas is heavier then air, so you really need to get it off the roof, or away from them units.

The vents were raised to about a foot and run down the middle of the roof.

That back vent in the pic looks close to that intake. I would temporarily raise the vents to 24" above your existing unit tops and see what happens after a few days or weeks. If the odor continues then oil the traps as stated by others. If the odors persist you might have an open cleanout behind a wall or open venting above a ceiling somewhere. Peppermint oil can help find the open pipe. Does your waste require storage and pumping?

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We have been in our building for 11 years. We have random strong sewer gas smells in the office which seem to happen on a no wind day. The roof has a flat top and our HVAC guys have been up there numerous times trying to figure out what is wrong.

I see your units have P traps but they don't go to anything. They are also not built right. The end of the P trap should be lower then the start of the P Trap. I do not know if the RTU fans are sucking air in through the P trap but it could compound your issue.